As power feed systems mentioned above is well known the one that is shown in FIG. 29 (refer to PTLs 1 and 2) for example. As shown in the figures, a power feed system 1 is provided with a power feed unit 3 as a power feed unit means, a power receiving unit 5 as a power receiving unit means. The aforementioned power feed unit 3 is provided with a power feed-side loop antenna 6 with which power is supplied, a power feed-side helical coil 7 as a power feed-side coil electromagnetically coupled with the power feed-side loop antenna 6. When power is supplied with the aforementioned power feed-side loop antenna 6, the power is transmitted to the power feed-side helical coil 7 by electromagnetic induction.
The aforementioned power receiving unit 5 is provided with a power receiving-side helical coil 9 electromagnetically resonated with the power feed-side helical coil 7, and a power receiving-side loop antenna 10 electromagnetically connected with the power receiving-side helical coil 9. When power is fed to the power feed-side helical coil 7, the power is wirelessly fed to the power receiving-side helical coil 9.
Furthermore, when power is fed to the power receiving-side coil 9, the power is fed to the power receiving-side loop antenna 10 by electromagnetic induction, supplied with a load that is connected with the power receiving-side loop antenna 10. According to the power feed system 1, it is made possible to feed power supplied from the power feed unit to the power receiving unit in the non-contact manner by electromagnetic induction between the power feed-side helical coil 7 and the power receiving-side helical coil 9.
It has been designed to feed power in the non-contact manner employing the aforementioned power feed system 1 to a load that is mounted in automobiles by providing the aforementioned power receiving unit 5 and the power feed unit 3 with the automobiles and such road, respectively.
A distance between the power receiving unit 5 mounted in the automobiles and the power feed unit 3, however, varies in accordance with types of the automobiles. Namely, an inter-coil distance L1 between the power receiving-side helical coil 9 of the power receiving unit 5 and the power feed-side helical coil 7 of the power feed unit 3 is varied in accordance with the types of the automobiles. For example, the inter-coil distance L1 becomes short when the power receiving unit 5 is mounted in automobiles with low body such as sports cars, whereas the inter-coil distance L1 becomes long when the power receiving unit 5 is mounted in automobiles with tall body such as station wagons.
The inventors then measured transit characteristics S21 and reflection characteristics S11 of the power receiving-side loop antenna 10 when the inter-coil distance L1 ranges from 100 millimeters (mm) to 400 mm while a radius of both the power feed-side loop antenna 6 and the power receiving-side loop antenna 10 of the power feed system 1 is fixed to 206 mm as shown in FIG. 29. The result is shown in FIGS. 30 to 32.
As apparent from the FIG. 30, fluctuation of the inter-coil distance L1 in the aforementioned power feed system 1 causes the transit characteristics S21 to vary. It is mainly caused, as found in FIGS. 31 and 32, by misalignment between the power feed-side helical coil 7 and the power receiving-side helical coil 9.
To explain more fully, when the radius of both the power feed-side loop antenna 6 and the power receiving-side loop antenna 10 is fixed to 206 mm, the inter-coil distance L1, when 200 mm, lies in nearly critical coupling, but shorter the inter-coil distance L1 becomes than that, the tightly the power feed unit 3 and the power receiving unit 5 is coupled, exhibiting bi-resonance characteristics, resulting in variation of frequency where transit characteristics becomes 1.
Accordingly, when operating frequency is fixed to nearly 13.5 megahertz (MHz), the transit characteristics S21, upon the inter-coil distance L1 being 200 mm, becomes nearly 1, leading to high efficiency, whereas the transit characteristics S21, if the inter-coil distance L1 shifting to 100 mm, decreases to nearly 0.64, causing increase of loss.
On the other hand, the longer the inter-coil distance L1, the looser the coupling between the power feed-side helical coil 7 and the power receiving-side helical coil 9 becomes, causing impedance matching to each other to uncouple, and thus loss to increase.
As can be seen from the above, there has been posed drawback that fluctuation of the inter-coil distance L1 makes power feeding efficiency from the power feed unit 3 to power receiving unit 5 vary, so as to increase the loss. Furthermore, there has been posed drawback that as shown in FIG. 9, when lateral shift x occurs that axis is shifted between the power feed-side loop antenna 6 or the power feed-side helical coil 7, and the power receiving-side loop antenna 10 or the power receiving-side helical coil 9, fluctuation of the inter-coil distance L1 also causes power feeding efficiency from the power feed unit 3 to power receiving unit 5 to vary, so as to increase the loss.